drm/amdgpu: Match against exact bootloader status

On PSP v13.x ASICs, boot loader will set only the MSB to 1 and clear the
least significant bits for any command submission. Hence match against
the exact register value, otherwise a register value of all 0xFFs also
could falsely indicate that boot loader is ready. Also, from PSP v13.0.6
and newer, bits[7:0] will be used to indicate command error status.

Signed-off-by: Lijo Lazar <lijo.lazar@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>